Failure when HTTP_PROXY doesn't have a scheme

[alexeagle]

my env had HTTP_PROXY=localhost:8888

and then subsequent builds fail with

ERROR: Error fetching repository: Traceback (most recent call last):
	File "/private/var/tmp/_bazel_alex.eagle/03c50bd9f989d25b88a1be300d53820c/external/rules_jvm_external/coursier.bzl", line 513, column 24, in _pinned_coursier_fetch_impl
		_add_outdated_files(repository_ctx, artifacts, repositories)
	File "/private/var/tmp/_bazel_alex.eagle/03c50bd9f989d25b88a1be300d53820c/external/rules_jvm_external/coursier.bzl", line 331, column 58, in _add_outdated_files
		"{proxy_opts}": " ".join(_get_java_proxy_args(repository_ctx)),
	File "/private/var/tmp/_bazel_alex.eagle/03c50bd9f989d25b88a1be300d53820c/external/rules_jvm_external/coursier.bzl", line 167, column 31, in _get_java_proxy_args
		return get_java_proxy_args(http_proxy, https_proxy, no_proxy)
	File "/private/var/tmp/_bazel_alex.eagle/03c50bd9f989d25b88a1be300d53820c/external/rules_jvm_external/private/proxy.bzl", line 39, column 37, in get_java_proxy_args
		proxy_user = _get_proxy_user(http_proxy)
	File "/private/var/tmp/_bazel_alex.eagle/03c50bd9f989d25b88a1be300d53820c/external/rules_jvm_external/private/proxy.bzl", line 2, column 33, in _get_proxy_user
		netloc = url.split("://", 1)[1]
Error: index out of range (index is 1, but sequence has 1 elements)

All this split logic needs to handle the case of a URI with no scheme https://github.com/bazelbuild/rules_jvm_external/blob/master/private/proxy.bzl#L2